Method and apparatus for providing antenna configuration information for massive multiple input multiple output in a wireless communication system

ABSTRACT

A method for transmitting a reference signal for supporting a plurality of antennas to a User Equipment (UE) at a Base Station (BS) in a wireless communication system is disclosed. The method includes grouping the plurality of antennas into antenna groups comprising a predetermined number of antenna ports, transmitting resource information about a reference signal for at least one antenna group of the antenna groups, and transmitting the reference signal for the at least one antenna group to the UE based on the resource information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2013/009045, filed on Oct. 10, 2013, which claims the benefit of U.S. Provisional Application No. 61/764,553, filed on Feb. 14, 2013, the contents of which are all hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for providing antenna configuration information for massive Multiple Input Multiple Output (MIMO) (i.e. MIMO with a plurality of antennas) in a wireless communication system.

BACKGROUND ART

A brief description will be given of a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system as an example of a wireless communication system to which the present invention can be applied.

FIG. 1 illustrates a configuration of an Evolved Universal Mobile Telecommunications System (E-UMTS) network as an exemplary wireless communication system. The E-UMTS system is an evolution of the legacy UMTS system and the 3GPP is working on the basics of E-UMTS standardization. E-UMTS is also called an LTE system. For details of the technical specifications of UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, respectively.

Referring to FIG. 1, the E-UMTS system includes a User Equipment (UE), an evolved Node B (eNode B or eNB), and an Access Gateway (AG) which is located at an end of an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and connected to an external network. The eNB may transmit multiple data streams simultaneously, for broadcast service, multicast service, and/or unicast service.

A single eNB manages one or more cells. A cell is set to operate in one of the bandwidths of 1.25, 2.5, 5, 10, 15 and 20 Mhz and provides Downlink (DL) or Uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be configured so as to provide different bandwidths. An eNB controls data transmission and reception to and from a plurality of UEs. Regarding DL data, the eNB notifies a particular UE of a time-frequency area in which the DL data is supposed to be transmitted, a coding scheme, a data size, Hybrid Automatic Repeat reQuest (HARQ) information, etc. by transmitting DL scheduling information to the UE. Regarding UL data, the eNB notifies a particular UE of a time-frequency area in which the UE can transmit data, a coding scheme, a data size, HARQ information, etc. by transmitting UL scheduling information to the UE. An interface for transmitting user traffic or control traffic may be defined between eNBs. A Core Network (CN) may include an AG and a network node for user registration of UEs. The AG manages the mobility of UEs on a Tracking Area (TA) basis. A TA includes a plurality of cells.

While the development stage of wireless communication technology has reached LTE based on Wideband Code Division Multiple Access (WCDMA), the demands and expectation of users and service providers are increasing. Considering that other radio access technologies are under development, a new technological evolution is required to achieve future competitiveness. Specifically, cost reduction per bit, increased service availability, flexible use of frequency bands, a simplified structure, an open interface, appropriate power consumption of UEs, etc. are required.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies on a method and apparatus for providing antenna configuration information to implement massive Multiple Input Multiple Output (MIMO) in a wireless communication system.

Technical Solution

The object of the present invention can be achieved by providing a method for transmitting a reference signal for supporting a plurality of antennas to a User Equipment (UE) at a Base Station (BS) in a wireless communication system, including grouping the plurality of antennas into antenna groups comprising a predetermined number of antenna ports, transmitting resource information about a reference signal for at least one antenna group of the antenna groups, and transmitting the reference signal for the at least one antenna group to the UE based on the resource information.

In another aspect of the present invention, provided herein is a method for receiving a reference signal for supporting a plurality of antennas from a BS at a UE in a wireless communication system, including receiving resource information about a reference signal corresponding to at least one of antenna groups comprising a predetermined number of antenna ports, the antenna groups being formed with the plurality of antennas, and receiving the reference signal for the at least one antenna group based on the resource information from the BS.

The resource information may include information indicating whether the at least one antenna group is a row or column of the plurality of antennas, or at least one of a row index and a column index of the plurality of antennas, for the at least one antenna group.

The antenna groups may be sub-arrays of the plurality of antennas, and the reference signal may be a Channel State Information-Reference Signal (CSI-RS).

The method may further include receiving polarization information about the at least one antenna group from the BS, and the polarization information may indicate at least one of a single polarization, a cross polarization and a multi-polarization. If the polarization information indicates the multi-polarization, the polarization information may further include information about a number of polarizations and polarization directions.

According to these methods, the UE may generate CSI using the reference signal for the at least one antenna group, and report the CSI to the BS.

It will be appreciated by persons skilled in the art that the effects that can be achieved with the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

Advantageous Effects

According to embodiments of the present invention, antenna configuration information for massive MIMO can be provided efficiently in a wireless communication system.

It will be appreciated by persons skilled in the art that that the effects that can be achieved through the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

In the drawings:

FIG. 1 illustrates a configuration of an Evolved Universal Mobile Telecommunications System (E-UMTS) network as an example of a wireless communication system;

FIG. 2 illustrates a control-plane protocol stack and a user-plane protocol stack in a radio interface protocol architecture conforming to a 3rd Generation Partnership Project (3GPP) radio access network standard between a User Equipment (UE) and an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN);

FIG. 3 illustrates physical channels and a general signal transmission method using the physical channels in a 3GPP system;

FIG. 4 illustrates a structure of a radio frame in a Long Term Evolution (LTE) system;

FIG. 5 illustrates a structure of a downlink radio frame in the LTE system;

FIG. 6 illustrates a structure of an uplink subframe in the LTE system;

FIG. 7 illustrates a configuration of a general Multiple Input Multiple Output (MIMO) communication system;

FIGS. 8 and 9 illustrate downlink Reference Signal (RS) configurations in an LTE system supporting downlink transmission through four antennas (4-Tx downlink transmission);

FIG. 10 illustrates an exemplary downlink Demodulation Reference Signal (DMRS) allocation defined in a current 3GPP standard specification;

FIG. 11 illustrates Channel State Information-Reference Signal (CSI-RS) configuration #0 of downlink CSI-RS configurations defined in a current 3GPP standard specification;

FIG. 12 illustrates antenna tilting schemes;

FIG. 13 is a view comparing an antenna system of the related art with an Active Antenna System (AAS);

FIG. 14 illustrates an exemplary AAS-based User Equipment (UE)-specific beamforming;

FIG. 15 illustrates an AAS-based two-dimensional beam transmission scenario;

FIG. 16 illustrates exemplary antenna array configurations for a two-dimensional array antenna structure;

FIG. 17 illustrates an exemplary signaling for configuring an antenna array according to an embodiment of the present invention;

FIG. 18 illustrates an exemplary signaling for configuring a sub-antenna array according to an embodiment of the present invention; and

FIG. 19 is a block diagram of a communication apparatus according to an embodiment of the present invention.

BEST MODE

The configuration, operation, and other features of the present invention will readily be understood with embodiments of the present invention described with reference to the attached drawings. Embodiments of the present invention as set forth herein are examples in which the technical features of the present invention are applied to a 3rd Generation Partnership Project (3GPP) system.

While embodiments of the present invention are described in the context of Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, they are purely exemplary. Therefore, the embodiments of the present invention are applicable to any other communication system as long as the above definitions are valid for the communication system. In addition, while the embodiments of the present invention are described in the context of Frequency Division Duplexing (FDD), they are also readily applicable to Half-FDD (H-FDD) or Time Division Duplexing (TDD) with some modifications.

The term ‘Base Station (BS)’ may be used to cover the meanings of terms including Remote Radio Head (RRH), evolved Node B (eNB or eNode B), Reception Point (RP), relay, etc.

FIG. 2 illustrates control-plane and user-plane protocol stacks in a radio interface protocol architecture conforming to a 3GPP wireless access network standard between a User Equipment (UE) and an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is a path in which the UE and the E-UTRAN transmit control messages to manage calls, and the user plane is a path in which data generated from an application layer, for example, voice data or Internet packet data is transmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transfer service to its higher layer, a Medium Access Control (MAC) layer. The PHY layer is connected to the MAC layer via transport channels. The transport channels deliver data between the MAC layer and the PHY layer. Data is transmitted on physical channels between the PHY layers of a transmitter and a receiver. The physical channels use time and frequency as radio resources. Specifically, the physical channels are modulated in Orthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL) and in Single Carrier Frequency Division Multiple Access (SC-FDMA) for Uplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, a Radio Link Control (RLC) layer via logical channels. The RLC layer at L2 supports reliable data transmission. RLC functionality may be implemented in a function block of the MAC layer. A Packet Data Convergence Protocol (PDCP) layer at L2 performs header compression to reduce the amount of unnecessary control information and thus efficiently transmit Internet Protocol (IP) packets such as IP version 4 (IPv4) or IP version 6 (IPv6) packets via an air interface having a narrow bandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (or L3) is defined only on the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration, reconfiguration, and release of radio bearers. A radio bearer refers to a service provided at L2, for data transmission between the UE and the E-UTRAN. For this purpose, the RRC layers of the UE and the E-UTRAN exchange RRC messages with each other. If an RRC connection is established between the UE and the E-UTRAN, the UE is in RRC Connected mode and otherwise, the UE is in RRC Idle mode. A Non-Access Stratum (NAS) layer above the RRC layer performs functions including session management and mobility management.

DL transport channels used to deliver data from the E-UTRAN to UEs include a Broadcast Channel (BCH) carrying system information, a Paging Channel (PCH) carrying a paging message, and a Shared Channel (SCH) carrying user traffic or a control message. DL multicast traffic or control messages or DL broadcast traffic or control messages may be transmitted on a DL SCH or a separately defined DL Multicast Channel (MCH). UL transport channels used to deliver data from a UE to the E-UTRAN include a Random Access Channel (RACH) carrying an initial control message and a UL SCH carrying user traffic or a control message. Logical channels that are defined above transport channels and mapped to the transport channels include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a Multicast Control Channel (MCCH), a Multicast Traffic Channel (MTCH), etc.

FIG. 3 illustrates physical channels and a general method for transmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, the UE performs initial cell search (S301). The initial cell search involves acquisition of synchronization to an eNB. Specifically, the UE synchronizes its timing to the eNB and acquires a cell Identifier (ID) and other information by receiving a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB. Then the UE may acquire information broadcast in the cell by receiving a Physical Broadcast Channel (PBCH) from the eNB. During the initial cell search, the UE may monitor a DL channel state by receiving a DownLink Reference Signal (DL RS).

After the initial cell search, the UE may acquire detailed system information by receiving a Physical Downlink Control Channel (PDCCH) and receiving a Physical Downlink Shared Channel (PDSCH) based on information included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources for signal transmission to the eNB, the UE may perform a random access procedure with the eNB (S303 to S306). In the random access procedure, the UE may transmit a predetermined sequence as a preamble on a Physical Random Access Channel (PRACH) (S303 and S305) and may receive a response message to the preamble on a PDCCH and a PDSCH associated with the PDCCH (S304 and S306). In the case of a contention-based RACH, the UE may additionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCH from the eNB (S307) and transmit a Physical Uplink Shared Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S308), which is a general DL and UL signal transmission procedure. Particularly, the UE receives Downlink Control Information (DCI) on a PDCCH. Herein, the DCI includes control information such as resource allocation information for the UE. Different DCI formats are defined according to different usages of DCI.

Control information that the UE transmits to the eNB on the UL or receives from the eNB on the DL includes a DL/UL ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPP LTE system, the UE may transmit control information such as a CQI, a PMI, an RI, etc. on a PUSCH and/or a PUCCH.

FIG. 4 illustrates a structure of a radio frame used in the LTE system.

Referring to FIG. 4, a radio frame is 10 ms (327200×T_(s)) long and divided into 10 equal-sized subframes. Each subframe is 1 ms long and further divided into two slots. Each time slot is 0.5 ms (15360×T_(s)) long. Herein, T_(s) represents a sampling time and T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33 ns). A slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMA symbols in the time domain by a plurality of Resource Blocks (RBs) in the frequency domain. In the LTE system, one RB includes 12 subcarriers by 7 (or 6) OFDM symbols. A unit time during which data is transmitted is defined as a Transmission Time Interval (TTI). The TTI may be defined in units of one or more subframes. The above-described radio frame structure is purely exemplary and thus the number of subframes in a radio frame, the number of slots in a subframe, or the number of OFDM symbols in a slot may vary.

FIG. 5 illustrates exemplary control channels included in a control region of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first one to three OFDM symbols of a subframe are used for a control region and the other 13 to 11 OFDM symbols are used for a data region according to a subframe configuration. In FIG. 5, reference characters R1 to R4 denote RSs or pilot signals for antenna 0 to antenna 3. RSs are allocated in a predetermined pattern in a subframe irrespective of the control region and the data region. A control channel is allocated to non-RS resources in the control region and a traffic channel is also allocated to non-RS resources in the data region. Control channels allocated to the control region include a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carrying information about the number of OFDM symbols used for PDCCHs in each subframe. The PCFICH is located in the first OFDM symbol of a subframe and configured with priority over the PHICH and the PDCCH. The PCFICH includes 4 Resource Element Groups (REGs), each REG being distributed to the control region based on a cell Identity (ID). One REG includes 4 Resource Elements (REs). An RE is a minimum physical resource defined by one subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4 according to a bandwidth. The PCFICH is modulated in Quadrature Phase Shift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ) indicator channel carrying an HARQ ACK/NACK for a UL transmission. That is, the PHICH is a channel that delivers DL ACK/NACK information for UL HARQ. The PHICH includes one REG and is scrambled cell-specifically. An ACK/NACK is indicated in one bit and modulated in Binary Phase Shift Keying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor (SF) of 2 or 4. A plurality of PHICHs mapped to the same resources form a PHICH group. The number of PHICHs multiplexed into a PHICH group is determined according to the number of spreading codes. A PHICH (group) is repeated three times to obtain a diversity gain in the frequency domain and/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDM symbols of a subframe. Herein, n is 1 or a larger integer indicated by the PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carries resource allocation information about transport channels, PCH and DL-SCH, a UL scheduling grant, and HARQ information to each UE or UE group. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, an eNB and a UE transmit and receive data usually on the PDSCH, except for specific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data and information indicating how the UEs are supposed to receive and decode the PDSCH data are delivered on a PDCCH. For example, on the assumption that the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked by Radio Network Temporary Identity (RNTI) “A” and information about data transmitted in radio resources (e.g. at a frequency position) “B” based on transport format information (e.g. a transport block size, a modulation scheme, coding information, etc.) “C” is transmitted in a specific subframe, a UE within a cell monitors, that is, blind-decodes a PDCCH using its RNTI information in a search space. If one or more UEs have RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicated by “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 6, a UL subframe may be divided into a control region and a data region. A Physical Uplink Control Channel (PUCCH) including Uplink Control Information (UCI) is allocated to the control region and a Physical uplink Shared Channel (PUSCH) including user data is allocated to the data region. The middle of the subframe is allocated to the PUSCH, while both sides of the data region in the frequency domain are allocated to the PUCCH. Control information transmitted on the PUCCH may include an HARQ ACK/NACK, a CQI representing a downlink channel state, an RI for Multiple Input Multiple Output (MIMO), a Scheduling Request (SR) requesting UL resource allocation. A PUCCH for one UE occupies one RB in each slot of a subframe. That is, the two RBs allocated to the PUCCH are frequency-hopped over the slot boundary of the subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocated to a subframe in FIG. 6.

Now a description will be given of a MIMO system. MIMO can increase the transmission and reception efficiency of data by using a plurality of Transmission (Tx) antennas and a plurality of Reception (Rx) antennas. That is, with the use of multiple antennas at a transmitter or a receiver, MIMO can increase capacity and improve performance in a wireless communication system. The term “MIMO” is interchangeable with ‘multi-antenna’.

The MIMO technology does not depend on a single antenna path to receive a whole message. Rather, it completes the message by combining data fragments received through a plurality of antennas. MIMO can increase data rate within a cell area of a predetermined size or extend system coverage at a given data rate. In addition, MIMO can find its use in a wide range including mobile terminals, relays, etc. MIMO can overcome a limited transmission capacity encountered with the conventional single-antenna technology in mobile communication.

FIG. 7 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 7, a transmitter has N_(T) Tx antennas and a receiver has N_(R) Rx antennas. The use of a plurality of antennas at both the transmitter and the receiver increases a theoretical channel transmission capacity, compared to the use of a plurality of antennas at only one of the transmitter and the receiver. The channel transmission capacity increases in proportion to the number of antennas. Therefore, transmission rate and frequency efficiency are increased. Given a maximum transmission rate R_(o) that may be achieved with a single antenna, the transmission rate may be increased, in theory, to the product of R_(o) and a transmission rate increase rate R_(i) in the case of multiple antennas. R_(i) is the smaller value between N_(T) and N_(R). R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was verified in the middle 1990s, many techniques have been actively proposed to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards such as standards for 3G mobile communications, future-generation Wireless Local Area Network (WLAN), etc.

Concerning the research trend of MIMO up to now, active studies are underway in many aspects of MIMO, inclusive of studies of information theory related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring MIMO radio channels and MIMO modeling, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.

Communication in a MIMO system with N_(T) Tx antennas and N_(R) Rx antennas as illustrated in FIG. 7 will be described in detail through mathematical modeling. Regarding a transmission signal, up to N_(T) pieces of information can be transmitted through the N_(T) Tx antennas, as expressed as the following vector. s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

A different transmission power may be applied to each piece of transmission information, s₁, s₂, . . . , s_(N) _(T) . Let the transmission power levels of the transmission information be denoted by P₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmission power-controlled transmission information vector is given as ŝ==[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P _(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector ŝ may be expressed as follows, using a diagonal matrix P of transmission power.

$\begin{matrix} {\hat{s} = {{\begin{bmatrix} P_{1} & \; & \; & 0 \\ \; & P_{2} & \; & \; \\ \; & \; & \ddots & \; \\ 0 & \; & \; & P_{N_{T}} \end{bmatrix}\begin{bmatrix} s_{1} \\ s_{2} \\ \vdots \\ s_{N_{T}} \end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) may be generated by multiplying the transmission power-controlled information vectors ŝ by a weight matrix W. The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel states, etc. These N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) are represented as a vector X, which may be determined by [Equation 5]. Herein, w_(ij) denotes a weight between a j^(th) piece of information and an i^(th) Tx antenna and W is referred to as a weight matrix or a precoding matrix.

$\begin{matrix} {x = {\quad{\begin{bmatrix} x_{1} \\ x_{2} \\ \vdots \\ x_{i} \\ \vdots \\ x_{N_{T}} \end{bmatrix} = {{\begin{bmatrix} w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\ w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\ \vdots & \; & \ddots & \; \\ w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\ \vdots & \; & \ddots & \; \\ w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}} \end{bmatrix}\begin{bmatrix} {\hat{s}}_{1} \\ {\hat{s}}_{2} \\ \vdots \\ {\hat{s}}_{j} \\ \vdots \\ {\hat{s}}_{N_{T}} \end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack \end{matrix}$

In general, the rank of a channel matrix is the maximum number of different pieces of information that can be transmitted on a given channel, in its physical meaning. Therefore, the rank of a channel matrix is defined as the smaller between the number of independent rows and the number of independent columns in the channel matrix. The rank of the channel matrix is not larger than the number of rows or columns of the channel matrix. The rank of a channel matrix H, rank(H) satisfies the following constraint. rank(H)≦min(N _(T) ,N _(R))  [Equation 6]

A different piece of information transmitted in MIMO is referred to as ‘transmission stream’ or shortly ‘stream’. The ‘stream’ may also be called ‘layer’. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of different pieces of transmittable information. Thus, the channel matrix H is determined by # of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 7]

“# of streams” denotes the number of streams. One thing to be noted herein is that one stream may be transmitted through one or more antennas.

One or more streams may be mapped to a plurality of antennas in many ways. The stream-to-antenna mapping may be described as follows depending on MIMO schemes. If one stream is transmitted through a plurality of antennas, this may be regarded as spatial diversity. When a plurality of streams are transmitted through a plurality of antennas, this may be spatial multiplexing. Needless to say, a hybrid scheme of spatial diversity and spatial multiplexing in combination may be contemplated.

It is expected that the future-generation mobile communication standard, LTE-A will support Coordinated Multi-Point (CoMP) transmission in order to increase data rate, compared to the legacy LTE standard. CoMP refers to transmission of data to a UE through cooperation from two or more eNBs or cells in order to increase communication performance between a UE located in a shadowing area and an eNB (a cell or sector).

CoMP transmission schemes may be classified into CoMP-Joint Processing (CoMP-JP) called cooperative MIMO characterized by data sharing, and CoMP-Coordinated Scheduling/Beamforming (CoMP-CS/CB).

In DL CoMP-JP, a UE may instantaneously receive data simultaneously from eNBs that perform CoMP transmission and may combine the received signals, thereby increasing reception performance (Joint Transmission (JT)). In addition, one of the eNBs participating in the CoMP transmission may transmit data to the UE at a specific time point (Dynamic Point Selection (DPS)).

In contrast, in downlink CoMP-CS/CB, a UE may receive data instantaneously from one eNB, that is, a serving eNB by beamforming.

In UL CoMP-JP, eNBs may receive a PUSCH signal from a UE at the same time (Joint Reception (JR)). In contrast, in UL CoMP-CS/CB, only one eNB receives a PUSCH from a UE. Herein, cooperative cells (or eNBs) may make a decision as to whether to use CoMP-CS/CB.

Channel State Information (CSI) reporting will be described below. In the current LTE standard, there are two MIMO transmission schemes, open-loop MIMO operating without channel information and closed-loop MIMO operating with channel information. Particularly in the closed-loop MIMO, each of an eNB and a UE may perform beamforming based on CSI to obtain a multiplexing gain through MIMO Tx antennas. To acquire CSI from the UE, the eNB may transmit a Reference Signal (RS) to the UE and may command the UE to feed back measured CSI on a PUCCH or PUSCH.

CSI is classified largely into three information types, RI, PMI, and CQI. An RI is information about a channel rank, as described before. The channel rank is the number of streams that a UE can receive in the same time-frequency resources. Because the RI is determined mainly according to the long-term fading of a channel, the RI may be fed back to an eNB in a longer period than a PMI and a CQI.

A PMI is the index of a UE-preferred eNB precoding matrix determined based on a metric such as Signal to Interference and Noise Ratio (SINR), reflecting the spatial characteristics of channels. A CQI represents a channel strength. In general, the CQI reflects a reception SINR that the eNB can achieve with a PMI.

An advanced system such as an LTE-A system considers achievement of an additional multi-user diversity through Multi-User MIMO (MU-MIMO). Due to the existence of interference channels between UEs multiplexed in an antenna domain in MU-MIMO, the accuracy of CSI may significantly affect interference with other multiplexed UEs as well as a UE that reports the CSI. Accordingly, more accurate CSI than in Single User MIMO (SU-MIMO) should be reported in MU-MIMO.

In this context, the LTE-A standard designs a final PMI separately as a long-term and/or wideband PMI, W1 and a short-term and/or subband PMI, W2.

For example, the long-term covariance matrix of channels expressed as [Equation 8] may be used for hierarchical codebook transformation that configures one final PMI with W1 and W2. W=norm(W1W2)  [Equation 8]

In [Equation 8], W2 is a short-term PMI, which is a codeword of a codebook reflecting short-term channel information, W is a codeword of a final codebook, and norm(A) is a matrix obtained by normalizing the norm of each column of matrix A to 1.

Conventionally, the codewords W1 and W2 are given as [Equation 9].

$\begin{matrix} {\mspace{79mu}{{{W\; 1\;(i)} = \begin{bmatrix} X_{i} & 0 \\ 0 & X_{i} \end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{{Nt}/2}\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.W}\; 2\;(j)} = {\overset{\overset{r\mspace{11mu}{columns}}{︷}}{\begin{bmatrix} e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\ \; & \; & \ldots & \; \\ {\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \; & {\gamma_{j}e_{M}^{m}} \end{bmatrix}}\mspace{14mu}\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},{{{where}\mspace{14mu} 1} \leq k},l,{m \leq {M\mspace{14mu}{and}\mspace{14mu} k}},l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack \end{matrix}$

In [Equation 9], the codewords are designed so as to reflect correlation characteristics between established channels, if cross-polarized antennas are arranged densely, for example, the distance between adjacent antennas is equal to or less than a half of a signal wavelength. The cross-polarized antennas may be divided into a horizontal antenna group and a vertical antenna group and the two antenna groups are co-located, each having the property of a Uniform Linear Array (ULA) antenna.

Therefore, the correlations between antennas in each group have the same linear phase increment property and the correlation between the antenna groups is characterized by phase rotation. Since a codebook is eventually quantized values of channels, it is necessary to design a codebook, reflecting channel characteristics. For the convenience of description, a rank-1 codeword designed in the above manner may be given as [Equation 10].

$\begin{matrix} {{W\; 1\;(i)*W\; 2\;(j)} = \begin{bmatrix} {X_{i}(k)} \\ {\alpha_{j}{X_{i}(k)}} \end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack \end{matrix}$

In [Equation 10], a codeword is expressed as an N_(T)×1 vector where NT is the number of Tx antennas and the codeword is composed of an upper vector X_(i)(k) and a lower vector α_(j)X_(i)(k), representing the correlation characteristics of the horizontal and vertical antenna groups, respectively. Preferably, X_(i)(k) is expressed as a vector having the linear phase increment property, reflecting the correlation characteristics between antennas in each antenna group. For example, a Discrete Fourier Transform (DFT) matrix may be used for X_(i)(k).

An advanced system such as an LTE-A system considers achievement of an additional multi-user diversity by the use of MU-MIMO. Due to the existence of interference channels between UEs multiplexed in an antenna domain in MU-MIMO, the accuracy of CSI may significantly affect interference with other multiplexed UEs as well as a UE that reports the CSI. Accordingly, more accurate CSI than in SU-MIMO should be reported in MU-MIMO.

In CoMP JT, because a plurality of eNBs transmit the same data to a specific UE through cooperation, they may be theoretically regarded as forming a MIMO system with antennas distributed geographically. That is, even when MU-MIMO is implemented in JT, highly accurate CSI is required to avoid interference between CoMP-scheduled UEs as in a single cell MU-MIMO operation. The same thing applies to CoMP CB. That is, to avoid interference with a serving cell caused by a neighbor cell, accurate CSI is needed. In general, a UE needs to report an additional CSI feedback in order to increase the accuracy of CSI feedback. The CSI feedback is transmitted on a PUCCH or a PUCSH to an eNB.

Now a detailed description will be given of RSs.

In general, a transmitter transmits RSs known to both the transmitter and a receiver along with data to the receiver so that the receiver may perform channel measurement in the RSs. The RSs indicate a modulation scheme for demodulation as well as the RSs are used for channel measurement. The RSs are classified into Dedicated RS (DRS) for a specific UE (i.e. UE-specific RS) and Common RS (CRS) for all UEs within a cell (i.e. cell-specific RS). The cell-specific RS includes an RS in which a UE measures a CQI/PMI/RI to be reported to an eNB. This RS is referred to as Channel State Information-RS (CSI-RS).

FIGS. 8 and 9 illustrate RS configurations in an LTE system supporting DL transmission through four antennas (4-Tx DL transmission). Specifically, FIG. 8 illustrates an RS configuration in the case of a normal CP and FIG. 9 illustrates an RS configuration in the case of an extended CP.

Referring to FIGS. 8 and 9, reference numerals 0 to 3 in grids denote cell-specific RSs, CRSs transmitted through antenna port 0 to antenna port 3, for channel measurement and data modulation. The CRSs may be transmitted to UEs across a control information region as well as a data information region.

Reference character D in grids denotes UE-specific RSs, Demodulation RSs (DMRSs). The DMRSs are transmitted in a data region, that is, on a PDSCH, supporting single-antenna port transmission. The existence or absence of a UE-specific RS, DMRS is indicated to a UE by higher-layer signaling. In FIGS. 8 and 9, the DMRSs are transmitted through antenna port 5. 3GPP TS 36.211 defines DMRSs for a total of eight antenna ports, antenna port 7 to antenna port 14.

FIG. 10 illustrates an exemplary DL DMRS allocation defined in a current 3GPP standard specification.

Referring to FIG. 10, DMRSs for antenna ports 7, 8, 11, and 13 are mapped using sequences for the respective antenna ports in a first DMRS group (DMRS Group 1), whereas DMRSs for antenna ports 9, 10, 12, and 14 are mapped using sequences for the respective antenna ports in a second DMRS group (DMRS Group 2).

As compared to CRS, CSI-RS was proposed for channel measurement of a PDSCH and up to 32 different resource configurations are available for CSI-RS to reduce Inter-Cell Interference (ICI) in a multi-cellular environment.

A different CSI-RS (resource) configuration is used according to the number of antenna ports and adjacent cells transmit CSI-RSs according to different (resource) configurations, if possible. Unlike CRS, CSI-RS supports up to eight antenna ports and a total of eight antenna ports from antenna port 15 to antenna port 22 are allocated to CSI-RS in the 3GPP standard. [Table 1] and [Table 2] list CSI-RS configurations defined in the 3GPP standard. Specifically, [Table 1] lists CSI-RS configurations in the case of a normal CP and [Table 2] lists CSI-RS configurations in the case of an extended CP.

TABLE 1 CSI Number of CSI reference signals configured reference 1 or 2 4 8 signal n_(s) n_(s) n_(s) Config- (k′, mod (k′, mod (k′, mod uration l′) 2 l′) 2 l′) 2 Frame 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 structure 1 (11, 2)  1 (11, 2)  1 (11, 2)  1 type 1 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 and 2 3 (7, 2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 0 6 (10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5) 1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3, 2) 1 15 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Frame 20 (11, 1)  1 (11, 1)  1 (11, 1)  1 structure 21 (9, 1) 1 (9, 1) 1 (9, 1) 1 type 2 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 only 23 (10, 1)  1 (10, 1)  1 24 (8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28 (3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

TABLE 2 CSI Number of CSI reference signals configured reference 1 or 2 4 8 signal n_(s) n_(s) n_(s) config- (k′, mod (k′, mod (k′, mod uration l′) 2 l′) 2 l′) 2 Fram 0 (11, 4)  0 (11, 4)  0 (11, 4) 0 structure 1 (9, 4) 0 (9, 4) 0  (9, 4) 0 type 1 2 (10, 4)  1 (10, 4)  1 (10, 4) 1 and 2 3 (9, 4) 1 (9, 4) 1  (9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 0 6 (4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4) 0 11 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 Frame 16 (11, 1)  1 (11, 1)  1 (11, 1) 1 structure 17 (10, 1)  1 (10, 1)  1 (10, 1) 1 type 2 18 (9, 1) 1 (9, 1) 1  (9, 1) 1 only 19 (5, 1) 1 (5, 1) 1 20 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, 1) 1 23 (7, 1) 1 24 (6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

In [Table 1] and [Table 2], (k′, l′) represents an RE index where k′ is a subcarrier index and l′ is an OFDM symbol index. FIG. 11 illustrates CSI-RS configuration #0 of DL CSI-RS configurations defined in the current 3GPP standard.

In addition, CSI-RS subframe configurations may be defined, each by a periodicity in subframes, T_(CSI-RS) and a subframe offset Δ_(CSI-RS). [Table 3] lists CSI-RS subframe configurations defined in the 3GPP standard.

TABLE 3 CSI-RS periodicity CSI-RS subframe offset CSI-RS-SubframeConfig T_(CSI-RS) Δ_(CSI-RS) I_(CSI-RS) (subframes) (subframes) 0-4 5 I_(CSI-RS)  5-14 10 I_(CSI-RS)-5 15-34 20 I_(CSI-RS)-15 35-74 40 I_(CSI-RS)-35  75-154 80 I_(CSI-RS)-75

Information about a Zero Power (ZP) CSI-RS is transmitted in a CSI-RS-Config-r10 message configured as illustrated in [Table 4] by RRC layer signaling. Particularly, a ZP CSI-RS resource configuration includes zeroTxPowerSubframeConfig-r10 and a 16-bit bitmap, zeroTxPowerResourceConfigList-r10. zeroTxPowerSubframeConfig-r10 indicates the CS-RS transmission periodicity and subframe offset of a ZP CSI-RS by I_(CSI-RS) illustrated in [Table 3]. zeroTxPowerResourceConfigList-r10 indicates a ZP CSI-RS configuration. The elements of this bitmap indicate the respective configurations written in the columns for four CSI-RS antenna ports in [Table 1] or [Table 2]. That is, the current 3GPP standard defines a ZP CSI-RS only for four CSI-RS antenna ports.

TABLE 4 -- ASN1START CSI-RS-Config-r10 ::= SEQUENCE {  csi-RS-r10 CHOICE { ...  }  zeroTxPowerCSI-RS-r10 CHOICE { release NULL, setup SEQUENCE {  zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE (16)),  zeroTxPowerSubframeConfig-r10 INTEGER (0..154) }  } } -- ASN1STOP

The current 3GPP standard defines modulation orders and cording rates for respective CQI indexes as illustrated in [Table 5].

TABLE 5 CQI code rate × index modulation 1024 efficiency 0 out of range 1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5 QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.9141 9 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 666 3.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

A CQI is calculated based on interference measurement as follows.

A UE needs to measure a Signal to Interference and Noise Ratio (SINR) for CQI calculation. In this case, the UE may measure the reception power (S-measure) of a desired signal in an RS such as a Non-Zero Power (NZP) CSI-RS. For interference power measurement (I-measure or Interference Measurement (IM)), the UE measures the power of an interference signal resulting from eliminating the desired signal from a received signal.

CSI measurement subframe sets C_(CSI,0) and C_(CSI,1) may be configured by higher-layer signaling and the subframes of each subframe set are different from the subframes of the other subframe set. In this case, the UE may perform S-measure in an RS such as a CSI-RS without any specific subframe constraint. However, the UE should calculate CQIs separately for the CSI measurement subframe sets C_(CSI,0) and C_(CSI,1) through separate I-measures in the CSI measurement subframe sets C_(CSI,0) and C_(CSI,1).

An Active Antenna System (AAS) and Three-Dimensional (3D) beamforming will be described below.

In a legacy cellular system, an eNB reduces ICI and increases the throughput of UEs within a cell, for example, SINRs at the UEs by mechanical tilting or electrical tilting, which will be described below in greater detail.

FIG. 12 illustrates antenna tilting schemes. Specifically, FIG. 12(a) illustrates an antenna configuration to which antenna tilting is not applied, FIG. 12(b) illustrates an antenna configuration to which mechanical tilting is applied, and FIG. 12(c) illustrates an antenna configuration to which both mechanical tilting and electrical titling are applied.

A comparison between FIGS. 12(a) and 12(b) reveals that mechanical tilting suffers from a fixed beam direction at initial antenna installation as illustrated in FIG. 12(b). On the other hand, electrical tilting allows only a very restrictive vertical beamforming due to cell-fixed tilting, despite the advantage of a tilting angle changeable through an internal phase shifter as illustrated in FIG. 12(c).

FIG. 13 is a view comparing an antenna system of the related art with an AAS. Specifically, FIG. 13(a) illustrates the antenna system of the related art and FIG. 13(b) illustrates the AAS.

Referring to FIG. 13, as compared to the antenna system of the related art, each of a plurality of antenna modules includes a Radio Frequency (RF) module such as a Power Amplifier (PA), that is, an active device in the AAS. Thus, the AAS may control the power and phase on an antenna module basis.

In general, a linear array antenna (i.e. a one-dimensional array antenna) such as a ULA is considered as a MIMO antenna structure. A beam that may be formed by the one-dimensional array antenna exists on a Two-Dimensional (2D) plane. The same thing applies to a Passive Antenna System (PAS)-based MIMO structure. Although a PAS-based eNB has vertical antennas and horizontal antennas, the vertical antennas may not form a beam in a vertical direction and may allow only the afore-described mechanical tilting because the vertical antennas are in one RF module.

However, as the antenna structure of an eNB has evolved to an AAS, RF modules are configured independently even for vertical antennas. Consequently, vertical beamforming as well as horizontal beamforming is possible. This is called elevation beamforming.

The elevation beamforming may also be referred to as 3D beamforming in that available beams may be formed in a 3D space along the vertical and horizontal directions. That is, the evolution of a one-dimensional array antenna structure to a 2D array antenna structure enables 3D beamforming. 3D beamforming is not possible only when an antenna array is planar. Rather, 3D beamforming is possible even in a ring-shaped 3D array structure. A feature of 3D beamforming lies in that a MIMO process takes place in a 3D space in view of various antenna layouts other than existing one-dimensional antenna structures.

FIG. 14 illustrates an exemplary UE-specific beamforming in an AAS. Referring to FIG. 14, even though a UE moves forward or backward from an eNB as well as to the left and right of the eNB, a beam may be formed toward the UE by 3D beamforming. Therefore, higher freedom is given to UE-specific beamforming.

Further, an outdoor to outdoor environment where an outdoor eNB transmits a signal to an outdoor UE, an Outdoor to Indoor (O2I) environment where an outdoor eNB transmits a signal to an indoor UE, and an indoor to indoor environment (an indoor hotspot) where an indoor eNB transmits a signal to an indoor UE may be considered as transmission environments using an AAS-based 2D array antenna structure.

FIG. 15 illustrates an AAS-based 2D beam transmission scenario.

Referring to FIG. 15, an eNB needs to consider vertical beam steering based on various UE heights in relation to building heights as well as UE-specific horizontal beam steering in a real cell environment where there are multiple buildings in a cell. Considering this cell environment, very different channel characteristics from those of an existing wireless channel environment, for example, shadowing/path loss changes according to different heights, varying fading characteristics, etc. need to be reflected.

In other words, 3D beamforming is an evolution of horizontal-only beamforming based on an existing linear one-dimensional array antenna structure. 3D beamforming refers to a MIMO processing scheme performed by extending to or combining with elevation beamforming or vertical beamforming using a multi-dimensional array antenna structure such as a planar array.

With a 2D array antenna structure, various antenna array configurations are possible.

FIG. 16 illustrates exemplary antenna array configurations for a 2D array antenna structure. In the illustrated case of FIG. 16, 8 antennas are given.

Referring to FIG. 16, 1×8, 2×4, 4×2, and 8×1 layouts are available as single polarization array configurations, and C(1×4), C(2×2), and C(4×1) layouts are available as cross polarization array configurations.

For the same number of antennas, the same CSI measurement and reporting procedure is performed in a cellular system such as the legacy LTE system. For example, since a 8-Tx PMI codebook is generated for a C(1×4) antenna configuration in the LTE system, even though an eNB has an 1×8 antenna configuration, a PMI is measured and fed back based on the same PMI codebook, that is, the 8-Tx PMI codebook.

While the use of the same codebook for the same number of antennas may not be feasible in terms of performance optimization, an increased codebook size increases signaling overhead and implementation complexity on a relatively large scale. Therefore, the increase of a performance gain brought by an increased codebook size is negligibly slight.

However, the introduction of the afore-described AAS may make various array configurations such as 2×4, 4×2, and C(2×2) available and may bring a substantial performance optimization gain. Particularly for 8 or more antennas, the performance optimization gain may be further increased.

As the afore-described 3D beamforming is available, a different feedback from conventional feedback information is required. For example, if a tall building is covered by 3D beamforming, a vertical-domain PMI (V-PMI) of a UE may change more slowly than a horizontal-domain PMI (H-PMI) of the UE.

In this case, the feedback overheads of the H-PMI and the V-PMI and the overheads of RSs with which the H-PMI and the V-PMI are measured may be controlled by allowing the UE to feed back the H-PMI and the V-PMI independently. One thing to note herein is that only when the UE has knowledge of an antenna array configuration of a transmitter, the UE may calculate the H-PMI and the V-PMI and distinguish a horizontal-domain CQI and RI (H-CQI/RI) from a vertical-domain CQI and RI (V-CQI/RI).

In this context, the present invention proposes that an eNB signals antenna array configuration information to a UE and the UE calculates and feeds back CSI based on the antenna array configuration information.

The antenna array configuration information may be transmitted to the UE by a physical-layer message or a higher-layer message. The physical-layer message or the higher-layer message may be a cell-specific signal or a UE-specific signal. Particularly when antenna array configuration information is transmitted by a UE-specific signal, it is possible to assign a sub-array on a UE basis.

The eNB may set a different number of effective antenna ports, a different layout of antenna ports, and/or different polarization information for each UE. The polarization information may indicate single polarization, cross polarization, or multi-polarization. In the case of three or more polarizations, that is, multiple polarizations, the polarization information may include information about a number of polarizations and polarization directions. The polarization information may also include information about a polarization type such as linear polarization or circular polarization.

Specifically, the antenna array configuration information may include information about an antenna port layout and/or polarization information. For example, given 8 antennas, 7 antenna configurations illustrated in FIG. 16 may be indexed. Then when an 8-Tx CSI-RS configuration is set by an RRC message, the index of the antenna configuration may be signaled along with CSI-RS resource information. If a plurality of physical antennas are bound into one antenna port, that is, antenna grouping is used, the antenna configuration information may be configuration information not from the perspective of a physical antenna configuration but from the perspective of antenna ports.

The antenna array configuration information may further include information indicating whether resources correspond to a row or a column in an antenna port configuration.

Specifically, if more antennas are used, a 2-D array may be configured using a plurality of CSI-RS resources (or CSI-RS patterns/CSI-RS processes).

FIG. 17 illustrates an exemplary signaling for configuring an antenna array according to an embodiment of the present invention.

Referring to FIG. 17, in a 32-Tx antenna environment with a 4×8 layout, four 8-Tx CSI-RS resources may be configured in each row of an antenna array and may be signaled respectively.

Regarding the CSI-RS resource configurations, it is necessary to indicate whether a CSI-RS resource configuration is for a row or column of the antennary array. This is because a CSI feedback for a row of an antenna array may be different from a CSI feedback for a column of an antenna array in terms of information configuration and a feedback period. Similarly, an RRC message for the CSI-RS resource configuration may further carry information indicating whether each CSI-RS resource is in the vertical domain or the horizontal domain, instead of the information indicating whether each CSI-RS resource corresponds to a row or column of an antenna array.

If a 2D array configuration is signaled using a plurality of CSI-RS resources, an RRC message for configuring CSI-RS resources may indicate a row or column of an antenna array to which each CSI-RS resource corresponds, as specific information of the antenna array configuration information.

Upon receipt of the above signal, a UE may calculate CSI for every row and column of the antenna array and feedback the CSI. For example, in a 4×8 32-Tx environment, an eNB may configure 8-Tx CSI-RS resources for each row of an antenna array and transmit information about the CSI-RS resource configuration. Then a UE may measure CSI for CSI-RS resources corresponding to each row of the antenna array and feed back the CSI to the eNB. That is, upon receipt of CSI-RS resources corresponding to the first to fourth rows of the antenna array, the UE may configure CSI for each row or a specific row of the antenna array and report an additional CSI feedback.

In this example, whether 8 Tx antennas correspond to a row or column of the antenna array does not matter to the UE because the UE may measure CSI corresponding to an opposite direction by stacking channels measured using four CSI-RS resources. For example, if channels are measured in a horizontal direction, V-CSI may be generated by interpreting a matrix with the horizontal channels stacked row by row, column by column. Likewise, if channels are measured in a vertical direction, H-CSI may be generated by interpreting a matrix with the vertical channels stacked row by row, column by column.

Information about a row or column of an antenna array may be extended to a row or column of a sub-array.

FIG. 18 illustrates an exemplary signaling for configuring a sub-antenna array according to an embodiment of the present invention.

Referring to FIG. 18, if antennas are arranged in a 8×8 64-Tx array, an eNB may set eight 8-Tx CSI-RS resources for a 2×4 sub-array structure and transmit (row, column) information about a corresponding sub-array. That is, the (row, column) information of a sub-array may be expressed as (1,1), (1,2), (2,1), (2,2), (3,1), (3,2), (4,1), and (4,2).

A UE may use antenna array configuration information in applying a PMI codebook optimized for an antenna array configuration indicated by the antenna array configuration information. In addition, the UE may use the antenna array configuration information in acquiring and feeding back other CSI required for a 2D array structure, for example, a V-PMI, a V-CQI/RI, an H-PMI, and an H-CQI/RI.

FIG. 19 is a block diagram of a communication apparatus according to an embodiment of the present invention.

Referring to FIG. 19, a communication apparatus 1900 includes a processor 1910, a memory 1920, an RF module 1930, a display module 1940, and a User Interface (UI) module 1950.

The communication device 1900 is shown as having the configuration illustrated in FIG. 19, for the convenience of description. Some modules may be added to or omitted from the communication apparatus 1900. In addition, a module of the communication apparatus 1900 may be divided into more modules. The processor 1910 is configured to perform operations according to the embodiments of the present invention described before with reference to the drawings. Specifically, for detailed operations of the processor 1910, the descriptions of FIGS. 1 to 18 may be referred to.

The memory 1920 is connected to the processor 1910 and stores an Operating System (OS), applications, program codes, data, etc. The RF module 1930, which is connected to the processor 1910, upconverts a baseband signal to an RF signal or downconverts an RF signal to a baseband signal. For this purpose, the RF module 1930 performs digital-to-analog conversion, amplification, filtering, and frequency upconversion or performs these processes reversely. The display module 1940 is connected to the processor 1910 and displays various types of information. The display module 1940 may be configured as, not limited to, a known component such as a Liquid Crystal Display (LCD), a Light Emitting Diode (LED) display, and an Organic Light Emitting Diode (OLED) display. The UI module 1950 is connected to the processor 1910 and may be configured with a combination of known user interfaces such as a keypad, a touch screen, etc.

The embodiments of the present invention described above are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by a subsequent amendment after the application is filed.

A specific operation described as performed by a BS may be performed by an upper node of the BS. Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with a UE may be performed by the BS, or network nodes other than the BS. The term ‘BS’ may be replaced with the term ‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘Access Point (AP)’, etc.

The embodiments of the present invention may be achieved by various means, for example, hardware, firmware, software, or a combination thereof. In a hardware configuration, the methods according to exemplary embodiments of the present invention may be achieved by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, etc.

In a firmware or software configuration, an embodiment of the present invention may be implemented in the form of a module, a procedure, a function, etc. Software code may be stored in a memory unit and executed by a processor. The memory unit is located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

The method and apparatus for providing antenna configuration information for massive MIMO in a wireless communication system have been described in the context of a 3GPP LTE system. Besides, the present invention is applicable to many other wireless communication systems. 

The invention claimed is:
 1. A method for transmitting a reference signal for supporting a plurality of antennas to a User Equipment (UE) at a Base Station (BS) in a wireless communication system, the method comprising: grouping the plurality of antennas into antenna groups comprising a predetermined number of antenna ports, wherein the antenna groups include a first antenna group and a second antenna group; transmitting first resource information on a first reference signal for the first antenna group and second resource information on a second reference signal for the second antenna group; transmitting polarization information about the antenna groups, wherein if a number of polarizations of the antenna groups is more than three, the polarization information includes information about the number of polarizations and polarization directions of the antenna groups; and transmitting the first reference signal and the second reference signal to the UE based on the first resource information and the second resource information, respectively.
 2. The method according to claim 1, wherein each of the first resource information and the second resource information includes information indicating whether each of the first antenna group and the second antenna group is a row or column of the plurality of antennas.
 3. The method according to claim 1, wherein each of the first resource information and the second resource information includes at least one of a row index and a column index of the plurality of antennas, for each of the first antenna group and the second antenna group.
 4. The method according to claim 1, wherein each of the first reference signal and the second reference signal is a Channel State Information-Reference Signal (CSI-RS).
 5. The method according to claim 1, wherein the plurality of antennas are arranged in an antenna array having 8 antenna rows and 8 antenna columns, and the plurality of antennas are grouped into 8 number of antenna groups.
 6. The method according to claim 5, wherein each of the 8 number of antenna groups has 2 antenna rows and 4 antenna columns.
 7. The method according to claim 1, further comprising: receiving, from the UE, a plurality of Channel State Information (CSI) reports generated using the first reference signal and the second reference signal, respectively.
 8. A method for receiving a reference signal for supporting a plurality of antennas from a Base Station (BS) at a User Equipment (UE) in a wireless communication system, the method comprising: receiving first resource information on a first reference signal for a first antenna group and second resource information on a second reference signal for a second antenna group, wherein the first antenna group and the second antenna group are included in antenna groups comprising a predetermined number of antenna ports and the plurality of antennas comprise the antenna groups; receiving polarization information about the antenna groups, wherein if a number of polarizations of the antenna groups is more than three, the polarization information includes information about the number of polarizations and polarization directions of the antenna groups; and receiving the first reference signal and the second reference signal from the BS based on the first resource information and the second resource information, respectively.
 9. The method according to claim 8, wherein each of the first resource information and the second resource information includes information indicating whether each of the first antenna group and the second antenna group is a row or column of the plurality of antennas.
 10. The method according to claim 8, wherein each of the first resource information and the second resource information includes at least one of a row index and a column index of the plurality of antennas, for each of the first antenna group and the second antenna group.
 11. The method according to claim 8, wherein each of the first reference signal and the second reference signal is a Channel State Information-Reference Signal (CSI-RS).
 12. The method according to claim 8, wherein the plurality of antennas are arranged in an antenna array having 8 antenna rows and 8 antenna columns, and the plurality of antennas are grouped into 8 number of antenna groups.
 13. The method according to claim 12, wherein each of the 8 number of antenna groups has 2 antenna rows and 4 antenna columns.
 14. The method according to claim 8, further comprising: generating a plurality of Channel State Information (CSI) using the first reference signal and the second reference signal, respectively; and reporting the plurality of CSI to the BS. 